Sensing and detecting low-level signals in the order of single photons or single electrons presents a challenging problem even today. In these sensors, the primary signals (optical, electrical, mechanical, chemical, radiation, etc.) are transformed into elementary charge carriers, such as electrons, holes, or ions, depending on the type of the device. Signal charge packets of such elementary charge carriers are amplified and converted to a signal (e.g., to a voltage signal) that generally can be detected and/or analyzed.
High-speed sensor devices with critical threshold parameters are in an acute demand in many applications, such as those relating to laser communication, deep space communication, quantum information processing, low-level signal imaging and other monitoring devices, recording and image transfer systems, and radiation or particle detecting systems. Such applications require sensors capable of detecting and recording electrical signals that are not only weak (e.g., as few as one or several elementary charge carriers), but also short in duration and/or rapidly varying (i.e., have a large bandwidth). Accordingly, these applications require a sensor capable of amplifying such electrical signals over a wide bandwidth and with a low noise level.
At present, generally the approach that is being followed in developing sensors is having signal amplification characteristics suited for detecting and recording weak electrical signals. There are limited alternatives to this approach.
Another approach to sensing weak electrical signals is using avalanche amplification (multiplication) of signal carriers, which generally is the most sensitive and high-speed method of amplification known. As is well known, avalanche amplification is based on impact ionization arising in a strong electric field, wherein the signal carriers accelerating in an electric field ionize the atoms of the working medium of the amplifier, thus resulting in multiplication (e.g., duplication) of the signal carriers. At a high multiplication factor, however, it is difficult to stabilize the avalanche amplification operating point. Additionally, the internal (excessive) noise level and the response time grow rapidly with increasing multiplication factor. Due to these problems associated with using a large multiplication factor, traditional avalanche photodiodes use a rather low multiplication factor, M, typically less than 100, that does not allow for detecting and recording signals consisting of several electrons in a wide band.
Avalanche multiplication has also been used for recording individual ionizing particles using a Geiger-Muller counter. A particle entering such a device initiates an avalanche-like process of multiplication of the signal carriers up to a necessary recording level. More recently, this principle has been successfully used for recording single charge carriers in semiconductor avalanche-type photodiodes, often referred to as Geiger-mode avalanche photodiodes (APDs). However, this Geiger-Muller principle of amplification operates above the breakdown voltage of the semiconductor. The breakdown voltage is defined in such a way that above the breakdown voltage the avalanche process becomes self-sustaining, resulting in a run-away avalanche that cannot be stopped by itself, theoretically approaching infinite avalanche gain. Because of the infinite avalanche, during this process a new arriving signal cannot be sensed unless the avalanche is stopped. Therefore, once a signal has been generated (sensed), it is necessary to stop the avalanche so that the photodiode can be reset to detect another signal. The avalanche process can be stopped by bringing the reverse bias voltage to below the breakdown voltage. This process of bringing the reverse bias voltage below the breakdown voltage is called “quenching”.
This quenching process even though faster does not bring the entire signal out of the avalanche process. It requires some time, called “dead time,” typically in the order of 1-10 μsec, to stop the avalanche process to an acceptable level so that the next signal can be detected.
In addition, known Geiger Mode Avalanche diodes do not allow for distinguishing between signals of one and several input charge carriers (i.e., it does not provide high resolution of the number of charge carriers).
Therefore, there remains a need for further advancements and improvements in detecting week signals, and particularly in providing a system and method for high sensitivity and high resolution detection of signals, as well as for such high resolution detection of weak signals with a high bandwidth (faster with no dead time, but only the reset time). Additionally, there remains a need for further advancements in Geiger-mode avalanche diodes that are sensitive to wavelengths of greater than about 950 nm, and particularly, there remains a need for Geiger-mode avalanche photodiodes that are sensitive to wavelengths of about 950 nm or greater and that have a high bandwidth (e.g., low reset time) and/or high resolution (e.g., single-photon resolution).